1. Field of the Invention
The invention is directed to multi-channel communication systems and in particular to methods for equalizing WDM systems.
2. Background Art
High capacity optical transmission networks, such as those defined by the SONET/SDH standards, can use wavelength-division multiplexing (WDM) to increase the information carrying capacity of the optical fiber. In optical WDM systems, a plurality of optical signals, each at a different wavelength, are transmitted over a single optical fiber. The transmitter terminal consists of a like plurality of optical transmitters, typically semiconductor lasers, and an optical wavelength multiplexer, which combines all optical signals into a multi-channel signal before it is launched over the optical fiber. Each transmitter operates at a different wavelengths and is modulated with a different data signal, either by directly modulating the laser or by external optical modulation.
At the receiver terminal, an optical wavelength demultiplexer separates the light received over the fiber according to the wavelength. The signal transmitted on each wavelength is then detected by a respective optical receiver.
The WDM system reach, or the distance between the transmitter and receiver sites, is limited by the attenuation or dispersion of the signal along the optical fiber. The reach can be increased by placing optical amplifiers at intermediate points between the terminals. Examples of optical amplifiers are semiconductor optical amplifiers, and rare earth doped fiber amplifiers. Optical amplifiers simultaneously amplify all optical signals passing through it, i.e. the multi-channel signal, by amplifying the optical power by a gain.
Unfortunately, optical amplifiers exhibit a wavelength-dependent gain profile, noise profile, and saturation characteristics. Hence, each optical signal experiences a different gain along the transmission path. The amplifiers also add noise to the signal, typically in the form of amplified spontaneous emission (ASE), so that the optical signal-to-noise ratio (OSNR) decreases at each amplifier site. The OSNR is defined as the ratio of the signal power to the noise power in a reference optical bandwidth.
Furthermore, the optical signals in the co-propagating channels have different initial waveform distortions and undergo different additional distortions during propagation along the fiber. As a result, the signals have different power levels, OSNRs, and degrees of distortion when they arrive at the respective receivers, if they had equal power levels at the corresponding transmitters.
WDM networks, and particularly SONET/SDH WDM networks, are widely spread and the custom demand for these networks is growing fast. They provide faster bit rates, and are more flexible in terms of the bandwidth per channel and complexity than the previous single-channel systems. Network providers are looking for features such as user-friendly installation, operation and maintenance, and thus, an equalization procedure that is simple and reliable will greatly simplify the set-up and hence reduce the maintenance costs of the communication system.
It has been shown that the OSNRs at the output of an amplified WDM system can be equalized by adjusting the input optical power for all channels. For example, U.S. Pat. No. 5,225,922 (Chraplyvy et al.), issued on Jul. 6, 1993 to AT&T Bell Laboratories, provides for measuring the output OSNRs directly and then iteratively adjusting the input powers to achieve equal OSNRs.
FIG. 1 shows a block diagram of a four-channel unidirectional wavelength division multiplexed (WDM) transmission link deployed between terminals 11 and 17, using OSNR equalization according to the above identified patent.
There are four unidirectional channels .lambda.(1)-.lambda.(4) illustrated on FIG. 1, carrying traffic in the direction West-to-East. A short discussion of this method follows for a better understanding of the present invention. Terminal 11 comprises transmitters T.sub.1 to T.sub.4 and terminal 17 comprises receivers R.sub.1 to R.sub.4, connected over optical amplifiers 10, 20, 30, 40 and 50 and fiber spans 10', 20', 30' and 40'. The optical amplifiers are arranged at a suitable distance from each other, typically 100 km, to compensate for the attenuation of the signal with the distance. An optical amplifier amplifies all four signals, as it is well known.
The lasers of the transmitters T.sub.1 to T.sub.4 are modulated with signals D.sub.1 to D.sub.4, respectively, to produce optical signals S.sub.1 to S.sub.4. A multiplexer 13 at the site of terminal 11, combines optical signals S.sub.1 -S.sub.4 into a multi-channel signal S, which is amplified in post-amplifier 10 before being launched over the transmission link. At reception, the multi-channel signal is amplified by pre-amplifier 50 and separated thereafter into signals S'.sub.1 -S'.sub.4 with demultiplexer 15. Each receiver at terminal 17 converts the respective optical signal into an output electrical signal D'.sub.1 -D'.sub.4, corresponding to input signals D.sub.1 to D.sub.4.
The U.S. Pat. No. 5,225,922 teaches establishing a telemetry link between two terminals 11 and 17 of a transmission network, for providing the measurements obtained at one terminal to the other. The patent indicates that the telemetry link may be provided with a control unit 5 (a microprocessor) that receives the measured input powers of signals S.sub.1 to S.sub.4 and the total output power or OSNR of multi-channel signal S, and adjusts the input power accordingly. This method also takes into account the known relative values of the gain for each channel. However, the method disclosed in the above patent has three disadvantages: (1) it equalizes OSNR, which is only one parameter of several that affect the performance of an optical transmission system, (2) measuring the OSNR requires additional equipment, such as an optical spectrum analyzer, outside of the SONET/SDH standards, and (3) it cannot be used to equalize systems where channels with different wavelengths carry traffic with different bit rates, since in such cases each channel has different OSNR requirements.
FIG. 1B shows the optical spectrum of a 4-channel WDM system, showing how the power of the channels varies with the wavelength.
As indicated above, in a typical WDM system the co-propagating channels do not have the same performance in terms of bit error rate (BER), because of different component losses, different transmitter and receiver characteristics, different path distortions, and also because the gain and noise of optical amplifiers in the system are channel-dependent. The BER is the ratio between the number of erroneous bits counted at a site of interest over the total number of bits received.
There is a need for providing a method for equalizing WDM systems that is more accurate and easier to implement than the current methods.